Change nodes position through scripts
Every node has it's own size/location properties that are accessible in the node itself.

import bpy
from mathutils import Vector

imgtx1 = nodes.new('ShaderNodeTexImage')
imgtx2 = nodes.new('ShaderNodeTexImage')

#location
imgtx1.location = Vector((200.0, 400.0))

#or
imgtx2.location.x = 100
imgtx2.location.y = 200

#size
# only width changes are allowed within the values of 
# node.bl_width_min and node.bl_width_max
imgtx2.width = 200
imgtx2.width_hidden = 100